Seals suitable for heat sealing around the mouth of a container for a commestible item may contain a metal foil layer, primarily used for induction heat sealable seals. The metal foil layer has on its underside heat sealable layers for adhering to the bottle mouth and, on the upper side, layers to provide mechanical strength and an upper surface which may be provided with print or other application of data. Seals in which a tab is provided lying wholly within the circumference of the seal have been described in, for instance, WO03/066465, U.S. Pat. No. 4,961,986 and U.S. Pat. No. 6,866,926. The tab may be formed by preventing lamination in a segment shaped portion of the seal, for instance by interleaving a tabstock material between a top, overall plastics layer and the underlying layer. An adhesive layer is positioned between the tabstock and the plastics layer, and continues between the plastics layer and the portion of the underlying layer extending beyond the tabstock.
Induction heating of the heat seal layer is often conducted after application of a cap, for instance a screw cap, to the neck of a container, the seal being pre-positioned within the cap. In order to provide a uniform pressure between the cap and the mouth of the container, so that the seal becomes adhered upon melting of the heat sealable layer, it is known to provide a resilient layer within the seal. This may be provided by a separate secondary liner, which is fixed in the cap above the seal e.g. formed of wadding material or foam. Alternatively it may be a layer within the seal itself positioned, for instance, between the metal foil and the top surface. Such a resilient layer may be formed of foamed polyolefin material, for instance.
There is an increasing attempt to recycle materials used in packaging consumer items, for instance bottles used for containing potable liquids. Polyethylene terephthalate is a convenient material widely used for such bottles, and processes have been developed for separating during recycling PET from other materials used in the production of containers, for instance materials used for the caps, labels and liners (or gaskets) for caps. PET has a density greater than 1 g/ml and is conveniently separate from other materials, which have a density less than 1, by a process involving aqueous baths, in which the PET sinks, and most other materials float. As well as separating labels and other particulate materials, the PET needs to be cleaned of residues of, for instance adhesives and rinsed, before being in a form suitable for reforming into containers or other end products by techniques involving melting the material. Several of the baths used in the recycling process tend to be highly caustic and are used at raised temperatures, for instance above 80° C. The materials remain in the baths for several minutes and are agitated therein. Under these conditions, we have found that aluminium exposed to the liquid may be rapidly degraded which reduces the life of the bath. Also, since one of the objectives of the bath is to remove deposits of adhesives, these baths often degrade adhesive layers used in the production of the laminates from which seals are produced. If aluminium degrades, then materials in adjacent layers tend to be released. Aluminium has a density of more than 1 g/ml. Depending on how it is incorporated into a seal, a chip containing an aluminium layer may sink with PET or float with other materials. We have appreciated that steps need to be taken to avoid chips containing aluminium sinking with the PET. The presence of residues of aluminium in PET being reused is very undesirable.